hepa filter efficiency after hydrogen fluoride exposureisnatt.org/conferences/31/19. bergman...

HEPA Filter Efficiency AfteryHydrogen Fluoride Exposure

Bechtel National, Inc., Richland, WA

Werner Bergman1 and Gerard Garcia

1 Aerosol Science LLC, Stanwood, WA

Work performed under contract to

Presented at

U.S. Department of EnergyHanford Tank Waste Treatment and Immobilization Plant Project

Presented at 31st Nuclear Air Cleaning Conference

July 19-21, 2010Charlotte NC

Hanford Tank Waste Treatment and Immobilization Plant Project

Charlotte, NC

1

OverviewObj ti•Objective:

•Develop a model that predicts HEPA filter aerosol penetration after exposure to HF

•Approach: • Derive an equation of HEPA filter penetration as a function of HF exposure and key filter parametersexposure and key filter parameters• Incorporate experimental values of filter fiber size and filter weights• Incorporate the HF reaction kinetics from experimental studiesp p

•Result:

2405 / T 1 3 Temperature (°K) Time (days)

PGl 0.003100 e13.74 0.7941.821e 2405 / T CHF, ppm,M

1.3 t

Penetration (%) HF Concentration (ppm)

Hanford Tank Waste Treatment and Immobilization Plant Project 2

Derivation of filter particle penetration modeldN NELD dTdN NELD dT

dTdN = decrease in number of particles in airN = number of particles

L

pD = fiber diameterL = fiber lengthE = efficiency of particle capture by fiberdT = slice of filter thickness, T

Adding all of the thickness slicesETLD

DN N0 e

ETLD


Derivation of filter particle penetration model, ContinuedFilter penetration, P, in % is

P N 100eETLD 100ekLDPN0

100e 100e

Where E and T are constant for a given filter and combined into k

For mixtures of different diameter and type of fibers we have

P 100 k LiDiP 100e i i

Adding the effect of media pinholes and defects yieldsg p y

P P0 100ek LiDi


Derivation of filter particle penetration model, ContinuedL i h l ti t di ( i ht l )• Lung inhalation studies (weight loss)

show acid exposure reduces the fiber diameter D0

D D0 2R t

D = fiber diameter, nmD0 = initial fiber diameter, nmR = etch rate, nm/dayt = time dayst time, days

• Equation for diameter reduction is consistent with DOP penetration

D

pmeasurements on HEPA media following HF exposure


Reduction in fiber diameter is more complexHF tt k lt i ti l l hi f l t l iHF attack results in a partial leaching of glass components leaving a more porous exterior before complete dissolution (From lung inhalation studies)

Original fiber

Porous partial reacted fiber

Unreacted fiber


Reduction in fiber diameter is more complexHF tt k d li id f th t f b d ll fib d•HF attack produces a liquid surface that forms beads on small fibers due

to surface tension.

•Larger diameter fibers do not have sufficient liquid to form beads•Larger diameter fibers do not have sufficient liquid to form beads.

• HEPA filter penetration measurements following HF exposure suggest the reduced diameter model yields satisfactory results.the reduced diameter model yields satisfactory results.


Derivation of filter particle penetration model, ContinuedAdding the decrease in fiber diameter with HF attack on glassAdding the decrease in fiber diameter with HF attack on glass fiber HEPA filters yields

P P 100ekLgl (Dgl ,02Rt )Pgl P0 100e gl gl ,0

If the HEPA filter also has acid resistant fibers, then

Pgl,A P0 100ek(LADA Lgl (Dgl ,02Rt ))gl, 0

The parameter values are obtained from experimental data


Determine fiber diameters and lengths for modelE i t l d t i id d f h d h t d f th HF t d• Experimental data is provided from hand sheets made for the HF study

• Diameter of fibers:• l fib di t bt i d f J h M ill BET d ti• glass fiber diameters are obtained from Johns Manville BET adsorption measurements (JM 110 glass = 2.7 m; JM 106 glass = 0.65 m)• asbestos and Nomex diameters are obtained from P and mass measurements

• Lengths of fibers:• are derived from fiber diameter and mass measurements

• Lack of mass data on filter hand sheets limits accuracy of these parameters• use mass data from H&V HEPA media to approximate missing data


Fiber diameter determined from pressure drop and mass measurements

P K i 1/ 2 i

Di2

K constant for filter mediaM /A i fib l f ti f t i i ii T

= fiber volume fraction of component i

Di fiber diameter of component iM = mass of filter media (not reported, used H & V data)A = area of filter mediaT = thickness of filter media (not reported, used H & V data)i density of filter fiber


Asbestos diameter estimated from pressure drop measurements

• Uncertainty range in fiber diameter and media weight defined by P equation• Uncertainty range in fiber diameter and media weight defined by P equation• Unique fiber diameter of 0.670 m determined when assuming H&V media weight.

1.6

Upper Size

1.2

1.4et

er,

m

P=33 mm

Upper Size

0.8

1

iber

Dia

me

P=35 mm

0.4

0.6

sbes

tos

Fi

P=37 mmLower size

0

0.2

As

H&V HEPA


70 72 74 76 78 80 82 84 86

Filter Medium M/A, g/m2

11

Nomex diameter estimated from pressure drop measurements• Uncertainty range in fiber diameter and media weight defined by P equationUncertainty range in fiber diameter and media weight defined by P equation

• Diameter range of 0.719-0.976 m determined when assuming average P and H&V media weight. Average diameter of 0.857 m has uncertainty.

1 4

1.2

1.4

m

5%10%15%Nomex

P mm H2O 35 39 45•Lack of single diameter shows assumption of constant filter mass is false

0.8

1

Dia

met

er,

m

0.4

0.6N

omex

D

39.7mm Avg. P

Lower size

0

0.2

50 60 70 80 90 100

5%

10%

15%


Filter Medium M/A, g/m2

12

Determination of fiber length•Fiber mass determined from H&V HEPA media mass and mass percent ofFiber mass determined from H&V HEPA media mass and mass percent of specific fiber

•Fiber diameter determined from P measurements

•Fiber length determined with the following formula

Li 4mi

D 2i Di


Determine HF reaction rate with glass fibers• No studies are found on the reaction rate of HF with glass fibers• No studies are found on the reaction rate of HF with glass fibers used in HEPA filters.

• The HEPA filter HF studies at Rocky Flats have only one HFThe HEPA filter HF studies at Rocky Flats have only one HF concentration and is not sufficient for determining reaction kinetics.

• The reaction kinetics of HF reaction with other glasses is obtained gfrom semi-conductor and inhalation toxicology studies. For the concentration region of interest, the rate is

Rate kCHF1.3

D t t th t HF i d i t l ti t h HEPA l•Data suggests that HF in vapor and in water solution etch HEPA glass fibers at the same rate


Etch rate of Glass varies widelyNote the log-log scale 100

Depends on:• Type of glass

HF i

Note the log-log scale

10

100

Mixed Oxide

• HF concentration• Need water (vap or liq)• Enhanced with acids• Rate = k C1 3 0 1

1

m/s

• Rate = k C1.3

0.01

0.1

ch R

ate,

nm

Vitreous

Experimental Data

Major Findings:•HEPA fibers are mixed

0.001Knotter (2000)Spierings (1993)Spierings (1987) 0 [HA]Spierings (1987) 2.9 [HA]Terada (1980) liquid

Etc Experimental DataHEPA fibers are mixed

oxides but have 1000 times slower etch rate.•Etch rate of HF vapor

d l ti i th

10-5

0.0001

0.01 0.1 1 10 100

Terada (1980) Gl-AsBrassell (1982) Gl-AsTerada (1980) GlBrassell (1982) Gl

JM 475Fiber

and solution is the same


Blow-up on next slide0.01 0.1 1 10 100

HF Concentration, mass %

15

Etch rate of HEPA glass fibers (JM 475 fibers)

• All data should fall on a single curve. The variation is due to

0.0002

Terada (1980) liquid

Experimental Data Vapor

missing filter mass and HF concentration data.

• The liquid etch point

0.00015Brassell (1982) (Gl-As)Terada (1980) (Gl-As)Terada (1980) (Gl)Brassell (1982) (Gl)Brassell (1982) Gl-Nomex)

/s• The liquid etch point concentration has large uncertainty. 0.0001

h R

ate,

nm

Liquid•The highest etch rates are used to be conservative. 5 10-5

Etch

JM 475Fiber

Liquid

00 0 05 0 1 0 15 0 2


0 0.05 0.1 0.15 0.2

HF Concentration, mass %

16

Experimental system used to expose glass HEPA filter media

Woodard et al , 1979


Experimental system used to expose glass HEPA filter mediaTh diff t t di d t d t R k Fl t HEPA di• Three different studies conducted at Rocky Flats on HEPA media exposure

to HF: Woodard et al 1978, Terada et al 1980, Brassell et al 1982

• Exposure consisted of spraying HF/water solution into a glove box having• Exposure consisted of spraying HF/water solution into a glove box having four 8” x 8” filter media sheets in the exhaust. The filter sheets were made as part of the study.

• All studies lacked key data for developing HF exposure model• HEPA media weight and fiber diameters not specified• HF challenge concentration either not stated or not reliable

•Approach for obtaining missing information:• Assume HEPA media weight is the same as commercial media• C t fib di t f P t• Compute fiber diameters from P measurements• Compute HF exposure from related measurements• Use the HF degradation of a reference media for estimating HF concentration (a litmus paper test for HF)


Determination of HF concentration from known solution concentration

• Sealed glove box ensures that the only source of water is from the acid spray

f h / f h• Use water from the HF/water spray as a tracer for the HF•Used relative humidity, temperature and air flow measurements to compute the water content, CW, in the chamber•Measured the HF content, CHF % mass, in the spray solutionMeasured the HF content, CHF % mass, in the spray solution

CHFC x106

ppmHF,m CHFCw x10100 air


Determination of HF concentration using Gl-As reference filters

(1) Develop the reference filter equation using a known HF concentration(1) Develop the reference filter equation using a known HF concentration

Pgl,5%As 0.0024 100 e0.3294 2.191 40.62 0.789 0.0004494 CHF,ppm,M

1.3 t

(2) Fit the reference equation to penetration data to obtain the

k HF t ti

0.2

Bras(Fig 5)

Bras(Fig 1)

Ter(Fig2)%unknown HF concentration 0.15( g )

Ter(Fig3)

netr

atio

n, %

PPM m,HF 16.2 (13.7)

0.1m

DO

P Pe (9.88) (9.63)known

conc.

0.05

0.3

unknownconc.


00 1 2 3 4 5 6 7

Exposure Time, Weeks20

Determination of HF concentration

HF concentrations derived from solution concentration or from degradationHF concentrations derived from solution concentration or from degradation of reference filter

Reference Filter Media HF Concentration

Reported, ppm Computed from solution, ppm

Computed from reference filter

d d tidegradation, ppm

Brassell Fig 1 5% asbestos 5 ppm 16.2 ppm ---Brassell Fig. 3 5% asbestos,

5% Nomex5 ppm 16.2 ppm ---

5% NomexBrassell Fig. 5 Glass,

5% asbestos2 ppm --- 9.88 ppm

Terada Fig. 2 Glass, 5% asbestos

--- --- 9.63 ppm

T d Fi 3 5% NTerada Fig. 3 5% Nomex,5% asbestos

--- --- 13.73 ppm


Addi th i t l t l i ld th HF filt d l

Putting it all together: adding experimental parameters

• Adding the experimental parameter values yields the HF filter model

Pgl, 5%As 0.0024 100 e 0.3294 2.191 40.62 0.789 0.0004494CHF, ppm,M

1.3 t gl, 5%As

• This equation predicts the DOP % penetration of a glass fiber HEPA media with 5% asbestos as a function of HF exposure at 70°F.

• Note, this equation applies to both media samples and any full-scale HEPA filter. There is no velocity dependence.

• This equation is used as a reference filter to determine the HF concentration in other tests.


Putting it all together: adding temperature effects Th hi i i h

R k1 Ea

C1 3

1

%

70°F

Ea = 20 kJ/molThe etching rate increases with temperature.

Rate kGl1 exp a

RT

CHF ,ppm ,M

1.3

•The Ea for the HEPA 0 6

0.8

netr

atio

n, % 100°F

150°F200°Fa

glass fibers is not known.

0.4

0.6

m D

OP

Pen

•An estimate based on mixed oxide glass is 20kJ/mol.

0.2

E t

0.3

00 1 2 3 4 5 6 7

Expt.

9.62 ppmM HF Exposure, Weeks


pp p

23

Models for glass, glass-asbestos and glass-Nomex filters Gl HEPA filtGlass HEPA filters

PGl 0.003100 e13.74 0.794 1.821e 2405 / T CHF, ppm,M

1.3 t

Glass-asbestos HEPA filters

PAsGl 0.0024 48.59 e13.38 0.7891.602e 2405 / T CHF, ppm,M

1.3 t

Glass-5% Nomex HEPA filters2405 / T 1 3 PN Gl 0.003 21.47 e

14.99 0.785 2.108e 2405 / T CHF, ppm,M1.3 t


General trends in HF filter model

0.12

0.14

Pen. ExperimentPen,theoryFiber Diameter16 2 ppm % HF reacted

12

14

(%)

cted

(%)

0.1

16.2 ppm % HF reacted

10

enet

ratio

n

HF

Rea

c

0.06

0.08

6

8

m D

OP

Pe

x10

(m

);

0.04 4

0.3 m

Dia

met

er x

0

0.02

0 5 10 15 20 250

2

Fibe

r D

glass fiber media5% asbestos


HF Exposure, days

25

Comparison of different HEPA filters to HF exposure A b t d N i HEPA lif li htl

1

Glass

0.1

Glass

Asbestos and Nomex increase HEPA life slightly

0.6

0.8

netr

atio

n, %

HF 10 ppm70°F

5% Nomex0.06

0.08

etra

tion,

% HF 10 ppm70°F

0.4

m D

OP

Pen

5% Asbestos0.04

0.06

m D

OP

Pen

5% Asbestos

0

0.2

0.3

0.02

0.3

5% Nomex

00 1 2 3 4 5 6 7 8

HF Exposure, Weeks

00 1 2 3 4 5 6

HF Exposure, Weeks


Conclusion • M d l d l d f di ti HEPA filt t ti f ll i• Models are developed for predicting HEPA filter penetration following HF exposure for glass, glass-asbestos and glass-Nomex fiber filters. The glass HEPA filter model is:

PGl 0.003100 e13.74 0.7941.821e 2405 / T CHF, ppm,M

1.3 t Temperature (°K) Time (days)

Gl

Penetration (%) HF Concentration (ppm)

• The accuracy of the models is based on a single HF concentration measurement and the following assumptions:

• mass of the media is the same as commercial HEPA media• the HF reaction rate for HEPA fibers is similar to the rate for other glasses = kCHF

1.3

•The temperature dependence is based on an activation energy of 20 kJ/mol for i d id l


mixed oxide glass.

27

hepa filter efficiency after hydrogen fluoride exposureisnatt.org/conferences/31/19. bergman...

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